Smectic liquid crystals are characterized by molecules which align in layers. Molecular ordering exists within each layer, with the degree of molecular ordering depending on the particular smectic phase. In chiral smectic materials, which include the chiral smectic C*, F*, I*, G* and H* phases, the ordering of the molecules within the layers rotates by a constant angle from layer to layer, so that the liquid crystal structure is twisted. For example, smectic C* phases are characterized in that the molecules align in layers in which the directors (i.e., the common directions of the long axes of the molecules within each layer) are oblique to the layer boundaries, so that the "tilt angle" of the molecules (i.e., the angle between the director and the layer normal) is the same from layer to layer. In bulk samples of smectic C* material, the directors twist from layer to layer.
Molecules of substances having chiral smectic liquid crystal phases have permanent dipole moments approximately normal to the director. The alignment of molecules of chiral smectic liquid crystal in an external electric field is determined in part by competition between the torques induced by this permanent dipole moment and by the induced dipole moment which, in materials with positive dielectric anisotropy, is parallel to the director. The smectic C*, F*, I*, G* and H* phases are all "ferroelectric" in the sense that geometry of the liquid crystal may be altered with the application of an electromagnetic field and then, under proper conditions, remains stable once the field is removed.
A number of transmissive mode displays using smectic C* phase liquid crystal have been proposed. For example, "surface-stabilized ferroelectric liquid crystal" ["SSFLC"] displays make use of thin films of smectic C* liquid crystal confined between parallel substrates. In SSFLC displays, the liquid crystal is preferably aligned in a so-called "bookshelf" geometry in which the molecules are arranged in layers that are perpendicular to the inner surfaces of the substrates and the molecules are approximately parallel along a 45.degree. angle to the layers. The material may be switched between two stable orientations by generating external electric fields across the liquid crystal normal to the inner surfaces of the substrates. The field useful for switching the material from one orientation to another is opposite in polarity to the field useful for switching back to the first of the orientations. If the film is sufficiently thin, each orientation should be stable when the electric field is removed. The two stable orientations differ in that the directors in the two orientations form mirror images about a plane normal to the layers and to the inner surfaces of the substrates.
In one proposed SSFLC device, the liquid crystal is confined between parallel substrates which are placed between a polarizer and an analyzer. The device modulates light by controlling the polarization direction of linearly polarized light transmitted through the liquid crystal. Due to the birefringence of the liquid crystal, light incident on the liquid crystal is decomposed into two orthogonally-polarized components having different speeds. The relative advance of one of the polarized components relative to the other rotates the polarization direction of the transmitted light relative to that of the incident light to a degree dependent on the thickness of the liquid crystal film and the orientation of the director within the film.
In a preferred device, the substrates contain a film of liquid crystal having a film thickness selected to effect a 90.degree. rotation of the polarization direction of the light incident on the liquid crystal. If the polarizer is aligned at approximately 45.degree. with respect to the optic axis of the liquid crystal in the first of the two orientations and the analyzer is oriented parallel or perpendicular to the polarizer, the light transmission through the device is maximized or minimized in the first stable orientation of the liquid crystal. Applying an electric field to transform the liquid crystal to the second stable orientation changes the intensity of light transmitted by the device. Consequently, such an SSFLC display is bright when the liquid crystal is in one of the stable orientations and dark when the liquid crystal is in the other orientation. The optical behavior of the device may be changed by rotating the analyzer relative to the polarizer, or either the polarizer or analyzer relative to the liquid crystal.
Among the advantages attributed to displays using smectic C* liquid crystal are fast switching times, high contrast and wide viewing angles compared to commercially available liquid crystal displays such as twisted nematic displays. One application for these ferroelectric liquid crystal materials would be in computer display terminals and televisions. Most currently available flat panel displays are based on twisted nematic liquid crystal. These flat panel displays require less power than conventional cathode ray tubes, but have not replaced cathode ray tubes due to their slow response, poor contrast, low brightness and narrow viewing angle.
Displays using ferroelectric smectic C* liquid crystal may be capable of switching times on the order of microseconds or less, whereas the switching times of twisted nematic displays are often on the order of milliseconds. Existing SSFLC displays have shown viewing angles in excess of 45.degree. and contrast ratios on the order of 1500:1, which exceed the performance of typical twist cells. Despite these advantages, SSFLC displays have not replaced cathode ray tubes due to the technical difficulty and expense of obtaining stable bookshelf alignment of the liquid crystal. Furthermore, the use of surface coatings with strong anchoring which promote alignment of the liquid crystal parallel to the substrates increases the switching voltage required to switch between stable orientations. Another disadvantage inherent in SSFLC displays is that the substrate anchoring that is necessary for bookshelf alignment is unstable; the liquid crystal may switch to light scattering "zig-zag" or "chevron" texture if jarred, rendering the display worthless. Despite intense research over the past decade, there remains a need for an economical method for aligning ferroelectric smectic liquid crystal in a geometry useful for display applications.
Flat panel liquid crystal displays using a nematic liquid crystal phase have been formed by phase separation of the liquid crystal phase from solution with a polymer or pre-polymeric resin. The earliest form of these materials comprised microdroplets of liquid crystal dispersed in a continuous polymeric matrix. In such materials, the ordinary index of refraction was matched to an index of refraction of the polymer. The material scattered light in the absence of an external field and transmitted light in the presence of an electric field. The evolution of such materials may be found in references such as U.S. Pat. Nos. 4,671,618; 4,673,255; 4,685,771; 4,688,900; 4,890,902; 5,004,323 and 5,093,735, the disclosures of which are incorporated by reference.
Three techniques have been proposed for inducing phase separation of the nematic liquid crystal phase from the polymer phase. According to a method known as "polymerization induced phase separation" or "PIPS," the liquid crystal is dissolved in a prepolymer followed by polymerization. According to another method known as "thermal induced phase separation" or "TIPS," the liquid crystal is dissolved (or redissolved) in a polymer melt followed by cooling. According to the third method, known as "solvent induced phase separation" or "SIPS," the liquid crystal and polymer are dissolved in a common solvent followed by evolution of the solvent. The polymer is often cross-linked to improve the properties of the display material. The size and density of the droplets may be varied by changing the ratios of the liquid crystal and polymer phases as well as by changing the conditions under which phase separation occurs.
While flat panel displays comprising material formed by phase separation of a nematic liquid crystal phase from solution with a polymer appear to be highly durable, as well as useful and economical for many applications, the fastest switching times for such materials remain on the order of a millisecond. Furthermore, the viewing angles for these materials can be increased beyond about 20.degree. only through the use of specialized birefringent polymers which increase the cost of the displays. There remains a need for relatively inexpensive flat panel displays with higher switching speeds, higher contrast and larger viewing angles.